Magnetic recording (“MR”) media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the magnetic domains of the grains of magnetic material.
A portion of a conventional longitudinal recording, thin-film, hard disk-type magnetic recording medium 1 commonly employed in computer-related applications is schematically illustrated in FIG. 1 in simplified cross-sectional view, and comprises a substantially rigid, non-magnetic metal substrate 10, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, having sequentially deposited or otherwise formed on a surface 10A thereof a plating layer 11, such as of amorphous nickel-phosphorus (Ni—P); a seed layer 12A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Ni—Al) or chromium-titanium (Cr—Ti) alloy; a polycrystalline underlayer 12B, typically of Cr or a Cr-based alloy; a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (“DLC”); and a lubricant topcoat layer 15, e.g., of a perfluoropolyether. Each of layers 11-14 may be deposited by suitable physical vapor deposition (“PVD”) techniques, such as sputtering, and layer 15 is typically deposited by dipping or spraying.
In operation of medium 1, the magnetic layer 13 is locally magnetized by a write transducer, or write “head”, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 13, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium layer 13 can subsequently produce an electrical response in a read transducer, or read “head”, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (“SMNR”) of the magnetic media. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, including that presented by the superparamagnetic limit, is to increase the crystalline anisotropy, thus the squareness of the magnetic bits, in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the writing head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. In this regard, it has been recently proposed (E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000) to provide a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled (“AFC”) by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability.
The strength of coupling can be described in terms of the total exchange energy. For a pair of ferromagnetic layers separated by a non-magnetic spacer layer, the total exchange energy generally results from RKKY-type interaction (i.e., oscillation from anti-ferromagnetic to ferromagnetic with increasing spacer film thickness), dipole-dipole interactions between grains of the ferromagnetic layers across the spacer layer (which favors anti-ferromagnetic alignment of adjacent grains across the spacer layer), and exchange interaction (which favors ferromagnetic alignment of the ferromagnetic layers). In AFC media the thickness of the spacer layer is chosen to maximize anti-ferromagnetic coupling between the ferromagnetic layers, i.e., to maximize the RKKY-type anti-ferromagnetic coupling and the dipole-dipole interactions. According to this approach, the total exchange energy between the ferromagnetic layer pairs is a key parameter in determining the increase in stability.
FIG. 2 schematically illustrates, in simplified cross-sectional view, a portion of an anti-ferromagnetically coupled (AFC) magnetic recording medium 20. As illustrated, medium 20 includes a non-magnetic substrate 10 selected from among non-magnetic metals and alloys, Al, Al-based alloys such as Al—Mg alloys, NiP-plated Al (NiP/Al), glass, ceramics, polymers, and composites of the aforementioned materials. The thickness of substrate 10 is not critical; however, in the case of magnetic recording media for use in hard disk applications, substrate 10 is of a thickness sufficient to provide the necessary rigidity. Substrate 10 typically comprises Al or an Al-based alloy, e.g., an Al—Mg alloy, and includes on the surface 10A thereof a plating layer 11, e.g., a layer of amorphous NiP. Formed on the plating layer 11 is an underlayer layer 12 for controlling the crystallographic texture and properties of ferromagnetic Co-based alloy layers deposited thereover, which underlayer 12 includes first, or lower, and second, or upper, portions 12A and 12B, respectively, as shown in FIG. 1, wherein the first, or lower portion 12A is a seed layer comprised of an amorphous or fine-grained material, e.g., a Ni—Al or Cr—Ti alloy layer from about 10 to about 1,000 Å thick, and the second, or upper portion 12B is a polycrystalline underlayer, typically a Cr or Cr-based alloy layer (e.g., of Cr—W, Cr—Mo, CoCr, etc.) from about 10 to about 300 Å thick.
In AFC medium 20, the single ferromagnetic layer 13 of the longitudinal magnetic recording medium 1 of FIG. 1 is replaced with a sandwich-type structure comprised of a pair of strongly anti-ferromagnetically coupled, crystalline ferromagnetic layers, i.e., a first, or lower, ferromagnetic layer 13L (alternatively referred to as a “bottom layer”) and a second, or upper, ferromagnetic layer 13U (alternatively referred to as a “top layer”), which pair of ferromagnetic layers are spaced-apart by at least one thin, crystalline, non-magnetic anti-ferromagnetic coupling (AFC) layer 16. Typically, each of the first, or lower, and second, or upper, ferromagnetic layers 13L and 13U, respectively, is comprised of an up to about 300 Å thick crystalline layer of at least one alloy of Co with at least one of Pt, Cr, B, Fe, Ta, Ni, Mo, V, Nb, Ru, Si, and Ge. The at least one thin, crystalline, non-magnetic spacer layer 16 is selected to provide a large RKKY-type coupling effect, and may comprise nearly any non-magnetic material, e.g., Ru, Ru-based alloys, Cr, and Cr-based alloys and is up to about 20 Å thick. Not shown in FIG. 2, for illustrative simplicity, is an optional magnetic layer, i.e., a top interface layer, interposed between the upper magnetic layer 13U and the non-magnetic spacer layer 16 for improving RKKY coupling between the upper and lower magnetic layers 13U and 13L.
High performance magnetic recording media generally require magnetic recording layers with well-defined, isolated magnetically isolated grains exhibiting little inter-granular magnetic coupling, i.e., segregated grains. Such recording layers with segregated grains typically comprise traditional hcp.lattice-structured CoCr-based alloys, wherein Co-based magnetic grains are magnetically isolated by Cr-rich (i.e., Cr-segregated) non-magnetic grain boundaries. CoCr-based alloys suitable for use in forming magnetic recording layers with segregated grains typically comprise CoCrTa or CoCrPtX alloys, where X is at least one element selected from Ta, B, Mo, V, Nb, W, Zr, Re, Ru, Ag, Hf, Ir, and Y.
In either instance, the added Ta and/or B primarily serve to improve the segregation of the Cr atoms in the magnetic layer. However, the segregation profile of the Cr atoms in the magnetic layer upon addition Ta and/or B to CoCr-based alloys is not necessarily sharp enough for media required to satisfy the ever-increasing performance criteria and parameters required of high-performance magnetic disk recording media utilized in computer-related applications.
Accordingly, there exists a need for improved magnetic recording media with sharper transitions (i.e., segregation profiles) between Co-containing magnetic grains and Cr-rich, non-magnetic grain boundaries, which improved media exhibit increased saturation magnetization (Ms) and magnetocrystalline anisotropy and narrower intrinsic switching field distribution.
The present invention, therefore, addresses and solves the need for increased Cr grain segregation in CoCr-based magnetic recording layers, leading to obtainment of improved magnetic recording media with enhanced performance characteristics, while maintaining full compatibility with all aspects of conventional automated manufacturing technology for fabrication of magnetic media, e.g., hard disks. Moreover, manufacture of the improved magnetic media of the present invention can be implemented at a cost comparable to that of existing media.